In the final scene when the survivors are preparing for hypersleep, there is an I.V. bag of "Sodium Chloride" solution by Bishop's head, milky white like the android's blood, not clear like a human's saline solution would be, with a printed date of "Expiry 07/87", the year after the movie was released.
After the first contact with the Aliens at the processing station and Drake is killed,Vasquez is being held back by Hicks near the door of the APC. He yells that Drake is gone and she says, "No he's not," but her mouth does not move in sync with the line.
When Ripley tries to break the glass in the med lab there is a scuff mark on the glass before her first swing. After this finishes, the mark disappears and we see Ripley actually making the mark with her second swing.
When Burke, Ripley and Lt. Gorman first enter the colony building, they pass through the pouring rain outside and get soaking wet. A few seconds later, inside the building, their clothes and Ripley's hair are dry again. (This scene is part of the extended edition)
Near the end when Ripley is on the dropship, she is seen arming herself. We see her grab a flamethrower from the weapons rack. She then lays a pulse rifle on the deck. Next she pulls a pulse rifle from the rack, and lays down a flamethrower. (The 2010 Blu-ray is slightly re-edited to remove this error.)
In the elevator escape scene where Hicks gets acid sprayed, he holds his gun in front of his face and turns his head to the left. Any acid burns he got would have been to the right side of his face. As they fight to remove his melting chest armor, there are no acid marks on his face. When they leave the elevator, there are acid burns are on the left side of his head, instead of the right side.
After an alien smashes a hole through the triangular windshield on the APC during the escape from under the cooling tower the windshield is then shown with an unbroken windshield as it exits on to the planet surface.
Towards the end, when Ripley is on her way to rescue Newt, we see the pilot light on Ripley's flamethrower. When she pauses to drop a flare there is no pilot light. The next shot shows the pilot light again.
The first dropship has the "Bugstomper: We endanger species" logo just beneath the cockpit. During the scene where the APC rolls around the corner and is loaded onto the dropship we see the other side (pointing to the right) of the dropship and the logo faces the same way as on the pointing to the left side. In every other scene the Bigstomper logo has been correctly painted in a reverse manner it still faces toward the front end of the dropship.
(In the extended version only) When Newt's parents explore the alien ship, there's a wide shot showing them pass through a gap in the hull. Next is an opposite shot showing them enter through the gap again.
When Gorman, Burke, and Ripley are discussing the firing of weapons under the primary heat exchangers as the marines are first going to sub level 3, Gorman's headset moves around on his hat, sometimes sitting below the rim of his hat and sometimes above the rim of his hat, finally toward the end it is completely under his hat.
As the Dropship heads to the planet, Cpl. Hicks is asleep. As the ship enters the atmosphere and the marines get ready to deploy, Hicks is clearly awake. In the next shot he is still asleep. Finally Apone says "Somebody wake up Hicks." (Extended edition)
The monitors for each of the marines' head-mounted cameras are identified by each marine's last name and first initial, located in lower right-hand corner of the monitor (e.g., Hicks D., Apone A., Hudson W., etc.). This is true for Vasquez' monitor (Vasquez J.), with one exception. While in the processing station and after Apone says, "Uh, roger. That's a two-one-six.", her monitor reads "Vasquez.".
In Alien (1979), the computer displays have an old-fashioned "teletype" clicking noise when they show data on their screens. However, when the screen in the shuttle comes to life at the beginning of Aliens, the display looks and sounds more modern like the other computer screens seen later in the film - even though 57 years have passed and the equipment in the shuttle could not have been updated during Ripley's hypersleep.
When the Marines and Ripley first encounter Newt, Drake fires wildly at Newt's scurrying silhouette as Hicks uses his own pulse rifle to deflect Drake's line of fire up and into the corridor wall. But there is no visible damage to the wall after Drakes ceases firing.
When Hicks first gives Ripley the locator wristband, the locator can be heard beeping at a steady rate. However, near the end of the film when Ripley finds the wristband while trying to rescue Newt. The locator beeps much faster, even though the wristband is about the same distance away as it was in the Ripley/Hicks scene earlier.
The shuttle pilot's seat has ejection handles. Since the shuttle travels between atmosphere & space, wouldn't ejection would be more dangerous than remaining on-board, both for the pilot AND passengers? Not necessarily so - pilot ejection would still be practical for atmospheric flight, and a sufficiently advanced seat could include a deployable bubble or shell for surviving space or even atmospheric re-entry - In the early 1960s, NASA designed a number of concepts for ejection seats that could be used to escape from a spacecraft in Earth orbit and allow an astronaut to safely re-enter the atmosphere. Passenger or payload recovery would be more problematic. But again, an integrated system of the future might provide reasonable survival capability.
When Ripley goes back to medical to join Newt, she lays her rifle on top of the bed and then lays down on the floor beside Newt. Moments later she looks to break the glass but finds her rifle laying on a table on the other side of the glass. The scene makes it clear that Burke entered while she and Newt were asleep and laid down the opened containment cylinders with the facehuggers inside of them; it's fairly obvious that he moved her rifle out of the room as well (note Ripley's expression when she sees her rifle).
The marines are supposed to be "Colonial Marines" but yet they have the patch of the American Flag on their uniforms (one of these patches is clearly visible on Gorman's uniform). Their full title is actually "US Colonial Marines" so an American flag would not be surprising.
When the salvage crew shines a light onto Ripley's face while she is sleeping at the beginning of the movie, her right eyelid twitches. However, people's eyelids twitch all the time when they are asleep.
When Bishop saves Newt from being sucked out of the airlock, part of Lance Henriksen's real torso can be seen beneath the false one, emerging from a hole in the floor. (This error was digitally corrected for the 2010 Blu-ray release.)
Just before Ripley rescues Newt from the cocoon, she kills the parasite coming out of the egg and one of the aliens charges at her. As it is moving down and towards Ripley, wires can be seen on top, helping to move it along.
During the escape in the APC, containers fall from the overhead storage, yet none of them strike Gorman anywhere near his head - they later say he has a concussion, and leaves the medlab with blood from his wounds showing through his bandage. In fact, only one of the containers strikes Gorman on the right arm and they way in which they fall indicate they are empty, clearly not enough force to knock him out cold.
Whilst Ferro is piloting the drop-ship down to the planetoid, on the computer screen showing navigation, the characters at the bottom of the screen are back to front, indicating that the scene had been mirrored.
During the scene in the queen's egg chamber shortly before Ripley "demonstrates" her flamethrower to the queen, an alien egg on the right of the screen, closest to Ripley's left leg, wobbles obviously. It's clearly very light and hollow.
After Spunkmeyer loads the missile, he calls "Clear behind!" and walks the power loader backward. A thick cable can be seen attached to the loader's left foot. (The loaders are supposed to be free-standing.)
The scenes showing the Marines inside the APC show that the vehicle is tall enough for them to stand upright inside of it. However, when the Marines exit the APC when inside of the atmosphere processor, it is obvious that they are quite a bit taller than the vehicle, revealing it to be a scale model.
When Ripley is preparing the M41 rifle with the flamethrower she is seen loading a new cartridge of ammo in the rifle as the counter runs up to 95. Once the elevator doors open the counter reads 42. She hasn't yet fired a shot.
The goof items below may give away important plot points.
When the queen grabs the loader in the final fight scene, she topples it over first smashing the yellow rotating beacon light on the top then pulling it in the airlock, but when the loader is shown in the air lock the light is undamaged and operating.
After the Queen falls down the airlock, the tip of her tail has been broken off (obviously unintentional, and overlooked or not repaired by the crew.) The tip is back on moments later as she's tumbling through space.
When Ripley is firing grenades into the Queen's ovipositor, after one of the last explosions, an intact, yellow chicken egg yolk is clearly seen sliding out of the wound. (It is even more obvious in one of the Quadrilogy's DVD featurettes.)
The wire that pulls the Queen's tail out of Bishop's chest is visible, but it is *not* the white strand that most people assume to be it. The strand is just an unidentified piece of material pulled out *by* the tail, and can be seen to snap and fall away. The wire itself is a monofilament visible for a brief moment before the tail begins to emerge.
When Vasquez shoots the alien which causes Drake's death, the exact shot of the alien exploding and spraying its acid blood everywhere is used again when the second pair of remote sentry guns (in the special edition) are firing away. In fact, it's used twice in this scene in quick succession.